The present invention relates to electrostatic assist gravure printing, and more specifically to an improved method and apparatus for applying a charge directly to the impression roll of a gravure press.
The basic electrostatic assist system for a gravure press is disclose in British Patent No. 1,159,923 published July 30, 1969, to Gravure Research Institute, Inc., entitled METHOD AND APPARATUS FOR TRANSFERRING INK IN GRAVURE PRINTING, the disclosure of which is incorporated herein by reference, and corresponding copending U.S. patent application Ser. No. 183,401 filed on Sept. 2, 1980, now U.S. Pat. No. 4,697,514, in the names of Harvey F. George and Robert H. Oppenheimer entitled, METHOD AND APPARATUS FOR TRANSFERRING INK IN GRAVURE PRINTING. The aforementioned British patent and the corresponding United States patent application disclose a semiconducting impression roll in which an electric charge is applied directly to the impression roll to create an electric field across the web at the nip between the gravure cylinder and the impression roll to cause the ink present in the gravure cells at the nip to more readily transfer to the web during printing, thereby minimizing the "skipped dots" problem. Further, it is disclosed in the aforementioned British patent and the corresponding United States patent application, that the charge can be applied directly to the inner metal core of the impression roll by a brush, indirectly via a corona wire spaced from the impression roll, or through a series of wire contacts in direct contact with an impression roll having a particular construction to provide a capacitive charging effect.
Various other charging arrangements based on those disclosed in the aforementioned British patent and United States patent application have also been used in electrostatic assist gravure printing for direct and indirect charging. Specifically, one such system for indirect charging uses a corona charging bar with ability to apply the charge over selected portions of the web to allow for changes in web width. See for example, British Patent No. 1,548,098 issued to Walter Spengler on July 4, 1979. However, such systems generally provide too little current and require very high voltage levels. Thus, they are more susceptible to press fires. Another system for indirect charging is disclosed in U.S. Pat. No. 4,208,965 issued to Eichler et al. on June 24, 1980, applies a corona charge through a plurality of decoupled electrodes to reduce the short circuit current to a value less than the critical breakdown current for the environment.
Another direct charging system uses a conductive roll which engages the impression roll to provide the direct application of charge thereto. Such a direct charging system suffers from the deficiency that the conductive roll must be quite large on a wide publication press, e.g., 9 inches or so in diameter, to avoid excessive deflection, and it is difficult to install, particularly on wide presses.
With indirect charging systems employing a corona producing means a high voltage of approximately 15,000 volts is needed to produce ions and electrons and drive them from the corona producing device to the impression roll. The corona current is about 400 microamperes per press unit maximum. However, with direct charging systems only up to about 2000 volts maximum is needed for paper and the systems presently used permit currents up to about 3 milliamperes before tripping. Further, the trip value can be set lower, as desired.
With the present direct charging arrangements, webs significantly narrower than the width of the impression roll create a number of problems. The current loss during charge application in non-web areas is significant and there is a gradual charge loss near the edges of the web thereby reducing the effectiveness of the electrostatic charging. One approach to solving this problem, is the undercutting of the impression roll to accommodate narrow webs. However, with deflection compensating impression rolls, such as the Bugel Roll manufactured by M.A.N. of Augsburg, West Germany; the CDR Controlled Deflection Roll manufactured by Motter Printing Press Co. of York, Pa.; the Flexible Impression Roller manufactured by Componenti Grafici of Lomellina, Italy; and the NIPCO Roller manufactured by Escher Wyss Ltd. of Zurich, Switzerland, undercutting of the impression roll to accommodate narrow webs is not a viable option. The NIPCO roller alleviates this problem somewhat by allowing pressure to be selectively applied over the area of the web with minimal pressure being applied to areas where the impression roll covering contacts the gravure cylinder directly. The remaining deflection compensating impression systems provide uniform impression pressure across the face width of the impression roll covering. However, all such deflection compensating impression systems result in significant current leakage during charge application.